Now that Map has some seriously complex deductions, it's about time
it had a command-line solver. In order to do this, I've had to
expose the internal region numbering because the solver has to have
some way to state which region it means; and in any case it's also
useful to have human-visible region numbering so that two people can
discuss a puzzle they're solving together. So pressing L during play
now toggles the display of region numbers; and `mapsolver' uses
those same numbers when showing its working and its solutions.

[originally from svn r6244]